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EARTH'S
MAGNETIC FIELD I am sure you are way ahead of me, particularly if you bothered to read the title: use a magnetic compass. As early as AD 1100 the Chinese knew that a pivoted magnet would point north (they said that it pointed south, but it is a matter of taste which end you choose to read). Since then the compass has been a valuable navigational aid and is still widely used despite the advent of sophisticated electronic and satellite equipment - a compass does not stop working when the power fails! The
catch is that a compass does not point exactly north. In most parts of the
world it is pretty close, but there some places in the Arctic and
Antarctic where it will indicate exactly the opposite direction, and at
the magnetic poles (one north of Canada and other just off the Antarctic
coast) it doesn't know which way to point. The solution is to have a chart
showing the angle between true north and magnetic north (this angle is
called the declination) for all parts of the world. The first such chart
was made in 1701 by Edmund Halley (1656-1742), the comet man, based
largely on his observations (see Figure 1). But there is another catch.
Declination changes slowly with time (Figure 2), so Halley's chart became
out of date and new ones had to be made. New charts are still being made
by the British and U.S. governments at 5-year intervals, for the benefit
of navigators and anyone else interested. |
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